Remove tests on unfiltered collection

These would be more appropiate in the backbone-proxy-collection repo.
Added tests to make sure ordering is preserved.

Added tests to make sure that if a model gets added at a specific index
then it will be put in the approriate place in the filtered proxy.
Added test for changing models
Implemented the previous tests

The filtered collection should now keep the same order of the superset
less the models that have been filtered out.